Field of the Invention
The invention relates to a bipolar junction transistor, and more particularly to a pseudo bipolar junction transistor using CMOS process.
2. Description of the Related Art
Variable grain amplifiers, auto-gain controllers, transfer linear function signal processors and recently-developed logarithmic filters and logarithmic current control oscillators are significantly applied in various fields. There is a common point for those circuits that bipolar junction transistors (BJTs) having an exponential function are used. Although in CMOS process, lateral transistors and MOS transistors operating in weak inversion region can be used for replacement, their application fields are limited due to their poor performances. Therefore, in many fields, BJTs can not be replaced with MOS transistors.
For variable gain amplifier and auto-gain controller applications, several pseudo-exponential circuits designed according to CMOS technology has been successfully developed. However, due to their complicated structures, the pseudo-exponential circuits can not function as BJTs. Therefore, CMOS transistors operating in saturation region are used to replace BJTs. The reason how come does the CMOS transistors operate in saturation region is that MOS transistors in weak inverse region has a narrow operation region, poor frequency response and poor match effect. Furthermore, lateral transistors have a small collector-base current gain and a larger leakage current. Consequently, making MOS transistors operate in saturation region has a valuable application.